Handbrake make mkv smaller

Hi,

I have rip my Blu-Ray with DVDFAB.
Each episode (23min) ends up with around 1.26 GB.

Then I've tried with handbrake using these settings but it still gets around 1.2 GB;
MKV Container
High Profile
Constant Quality 20 RF
Variable Framerate

How do I make it smaller without losing so much quality ??
When downloading series, it usually is around 550 MB/episode.
Here's how my files are:

Format: Matroska
Format version: Version 2
File size: 1.27 GiB
Duration: 22mn 49s
Overall bit rate mode: Variable
Overall bit rate: 7 945 Kbps
Writing application: DVDFab
Writing library: libebml v0.7.8 + libmatroska v0.8.1
Video
ID: 1
Format: AVC
Format/Info: Advanced Video Codec
Format profile: High@L4.1
Format settings, CABAC: Yes
Format settings, ReFrames: 2 frames
Codec ID: V_MPEG4/ISO/AVC
Duration: 22mn 49s
Bit rate mode: Variable
Bit rate: 7 338 Kbps
Maximum bit rate: 11.2 Mbps
Width: 1 920 pixels
Height: 1 080 pixels
Display aspect ratio: 16:9
Frame rate mode: Constant
Frame rate: 23.976 fps
Color space: YUV
Chroma subsampling: 4:2:0
Bit depth: 8 bits
Scan type: Progressive
Bits/(Pixel*Frame): 0.148
Stream size: 1.17 GiB (92%)
Default: Yes
Forced: No
Audio
ID: 2
Format: AC-3
Format/Info: Audio Coding 3
Mode extension: CM (complete main)
Format settings, Endianness: Big
Codec ID: A_AC3
Duration: 22mn 49s
Bit rate mode: Constant
Bit rate: 448 Kbps
Channel(s): 6 channels
Channel positions: Front: L C R, Side: L R, LFE
Sampling rate: 48.0 KHz
Bit depth: 16 bits
Compression mode: Lossy
Stream size: 73.1 MiB (6%)
Language: English
Default: Yes
Forced: No


Thanks in advance